In Ottawa–which is using the device on its police bikes–the information could also be helpful for deciding when and how to redesign roads to make them safer for cyclists.

In 28 states, it’s illegal to drive past someone on a bike without giving them at least three feet of space. Many drivers don’t know the laws exist. And it doesn’t matter, because the laws are rarely enforced–and hard to prove unless a cyclist is actually hit by a car (and sometimes not even then, if a driver can claim that it was the bike that swerved).

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“There is a learning curve for cyclists and drivers.”

That’s why some cities are starting to use the biking equivalent of a radar gun. A small gadget mounts to the handlebars on a police bike, sending out sonar that can measure the exact distance to a passing car. If someone veers too closely, the cop can give out a warning or a ticket.

After launching in Chattanooga, Tennessee, in 2015, the technology, called C3FT, is now in 11 cities, including Minneapolis, Houston, and Las Vegas. One of the latest is Ottawa, Canada.

“Like any large city, we have conflict between different modes of transportation on our roads,” says Kale Brown, project officer for Safer Roads Ottawa, a city program that helped introduce police to the new devices. “As we continue to grow and strengthen Ottawa’s network of cycling infrastructure, there is a learning curve for cyclists and drivers.”

Though Ottawa’s law went into effect in January, few drivers were aware of it. The sonar device is meant to change that. As police bike down city streets, the gadget beeps when someone drives closer than three feet. Then the officer radios ahead to another officer in a car, who pulls over the driver.

“We can look at if there’s something we can do to the physical design to change behavior.”

So far, police have only given warnings rather than tickets, which could range up to $110. “The device has been a great tool for us to educate drivers on proper behavior when around a cyclist,” says Brown. “We could be using the device for enforcement, but we’re looking to ensure the majority of residents are well informed of the law before that would begin . . . What we’ve seen in Ottawa is that this starts a conversation between cyclists and drivers.”

Ultimately, the gadgets may also help cities take a more important step: redesigning roads. Some argue that three-foot laws aren’t particularly helpful. Most of the laws come with caveats that let drivers off the hook, and even when drivers do learn about the laws, the fines are often so low that they may not act as much of a deterrent (in California, the base fine is $35, less than a parking ticket in L.A. or San Francisco). Others say that three feet isn’t enough distance to keep cyclists safe.

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When engineers in Austin first developed the device–as a side project for a bike nonprofit that helps novice riders get on the streets–their initial intent was to help cities design better bike lanes.

“The idea is that you ride on facilities with different designs, record statistics about driver behavior, and then you can feed that back,” says Christopher Stanton from Codaxus, the startup making C3FT. “If there are five designs on the road, and one design has 10 times as much close passing activity, rather than trying to do driver education, we can look at if there’s something we can do to the physical design to change behavior.”

That might mean getting rid of “sharrows” that put a bike lane in the middle of a car lane on some streets, or it might mean changing the placement of parked cars. Though none of the cities using the sonar device have started using it to plan redesigned streets yet, some are interested. Stanton hopes it leads to better roads.

“We’re doing this to try to change the built and policy environments,” he says.

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About the author

Adele Peters is a staff writer at Fast Company who focuses on solutions to some of the world's largest problems, from climate change to homelessness. Previously, she worked with GOOD, BioLite, and the Sustainable Products and Solutions program at UC Berkeley.